This invention relates to a lifting device with a higher speed in the downward movement area than in the upward movement area.
Lifting devices which pick up a load from a take-up zone and carry the load upward to a delivery area are well known in the art. Many of these devices include a load pick-up system which runs on an oval, circular, or loop-shaped track equipped with an upper and lower reversion device. The lifting device is driven by a driving motor and a gear. In addition, a brake is often provided which can stop the movement of the lifting device in the event of a power failure or an emergency stop.
It is well known in the art to use a "squirrel-cage" motor in the drive system. In a lifting device using such a conventional squirrel-cage motor, the driving speed is determined according to the permissible lifting speed under a load condition. When the drive system is working uniformly, th driving speed of the lifting device during its upward movement, i.e., when a load is being carried, is equal to that during its downward movement without a load.
It is preferable, however, for the downward movement of the lifting device to occur at a faster speed than the upward movement (the speed of which is limited by the load), thereby reducing the time of the lifting cycle.
Speed control of th conventional squirrel-cage motors has been accomplished in a number of ways. One such method is pole-changing, or altering the number of effective poles by regrouping the stator coils. Another method is frequency control, whereby the frequency of the power supply is varied by a control element. Yet another method accomplishes this objective by gear shifts.
The controlled speed adjustment thereby attained by these prior art methods referred to above is unfavorable because it is accomplished by a rigid, specified system which can only function by means of corresponding switching and control elements.
It is therefore an object of the present invention to provide a lifting device which reduces the time of the lifting cycle while avoiding the disadvantages of the change-over which is necessary in known devices.
It is another object of the present invention to provide a lifting device in which a speed is automatically achieved during the downward movement of the lifting device substantially caused by the weight of the load pick-up system acting a the driving power.
It is yet another objective of the present invention to provide a lifting system in which the speed of the upward movement of the lifting device is dependent upon the actual existing load.